1. Field of the Invention
The present invention relates to an optical fibre tensioning device and a method of controlling the tension applied to an optical fibre, particularly during the winding of the fibre.
2. Description of the Related Art
Typically, optical fibres are formed in draw towers from preforms of glass material. A draw tower is generally provided with a traction device (xe2x80x9ccapstanxe2x80x9d), for example a motorized pulley, capable of drawing the optical fibre downwards and feeding it to a storage device; the latter usually comprises a motorized reel onto which the optical fibre is wound.
In many operating situations, the speed at which the optical fibre is drawn by the traction device (the drawing speed) may not coincide with the speed at which the optical fibre is fed into the storage device, for example owing to fluctuations of the speed of the motors which drive the traction device and the storage device. The difference between these speeds gives rise to variations of tension in the fibre with respect to a target tension; in particular, a fibre tension greater than the target tension may damage the fibre, while a tension lower than the target tension may make the storage process inefficient.
To solve this problem, there is a known practice of using tensioning devices which are to be interposed between the traction device and the storage device and are capable of controlling the tension of the optical fibre. In particular, there are known tensioning devices comprising at least one fixed pulley and one movable pulley which can guide the optical fibre. The movement of the movable pulley enables the tension of the optical fibre to be modified. Here and in the remainder of the description, the term xe2x80x9cfixed pulleyxe2x80x9d denotes a pulley which is free to rotate about its axis and in which the position of the axis is fixed, while the term xe2x80x9cmovable pulleyxe2x80x9d denotes a pulley which is free to rotate about its axis and in which the position of the axis is variable.
U.S. Pat. No. 4,712,866 describes a tensioning device positioned between a traction device of a draw tower and a storage device comprising a take-up reel. This tensioning device comprises a pair of fixed pulleys and a movable pulley located on an oscillating arm, also called a xe2x80x9cdancer armxe2x80x9d, which carries a weight. The dancer arm actuates a conventional speed control so that the fibre is wound onto the take-up reel at a tension determined by the aforesaid weight.
U.S. Pat. No. 5,790,292 relating to an optical fibre transmission line describes, among other things, a draw tower in which a tensioning device with two pulleys is interposed between a traction device and a take-up reel (FIG. 7).
U.S. Pat. No. 4,138,069 describes a winding device for optical glass filaments in which a traction device draws the filament from a source of molten glass and feeds it through a tensioning device to a set of take-up reels. The tensioning device comprises two fixed pulleys and one movable pulley mounted on one end of a dancer arm. The dancer arm extends from an apparatus in which a spring or other means applies a constant force to move the movable pulley away from the fixed pulleys.
Tensioning devices can also be used in apparatus other than draw towers.
U.S. Pat. No. 5,076,104 describes an apparatus for measuring the load at failure of an optical fibre, in which tensioning devices comprising a pair of fixed pulleys and one movable pulley are provided. Also provided is a tensioning device comprising one fixed grooved wheel and one movable grooved wheel; the latter is positioned below the fixed grooved wheel, is movable in a vertical direction and is connected to a predetermined weight. The optical fibre, which in its path is wound around both the fixed grooved wheel and the movable grooved wheel, is thus kept at a constant tension.
U.S. Pat. No. 4,505,222 describes an extrusion coating device for optical fibres, in which a tensioning device is positioned immediately upstream of a take-up reel.
U.S. Pat. No. 4,127,370 describes an extrusion coating apparatus for an optical fibre, in which a first, a second and a third pulley are positioned at the entry to the apparatus, immediately downstream of the fibre take-off reel. The first and second pulley are spaced apart from each other and from the take-off reel. The third pulley has no support and is carried on the optical fibre between the first and second pulley, in such a way that the fibre passes, as a result of the weight of the third pulley, along a U-shaped path between the first and second pulley. A pair of photocells is used to detect the vertical position of the third pulley and to control the speed of rotation of the take-off reel in such a way that the vertical position of the third pulley, and therefore the tension of the fibre, remains essentially constant.
The applicant has observed that tensioning devices of known types, requiring the use of gravity and/or predetermined elastic action for controlling the position of the movable pulley, apply an uncontrollable predetermined force to the movable pulley. Therefore it is not possible to modulate in any way the force applied to the movable pulley in order to modify the response of the tensioning device. Consequently, the tension control obtained by means of the known devices, being of the non-modulatable type, might be inadequate in some operating conditions.
The applicant has found that the aforesaid problem can be at least partially overcome by using an optical fibre tensioning device in which there is a movable pulley on which the optical fibre runs, a device for measuring the tension applied to the optical fibre and a device for moving the movable pulley, capable of controlling the position of the movable pulley in accordance with the signal generated by the tension measuring device.
The tensioning device according to the present invention is capable of receiving at its input an optical fibre from a fibre guide or return element or from a device of another type, for example a traction device or a take-off device, and is capable of supplying the optical fibre from its output, after having automatically controlled its tension, to a further guide or return element, or to a further device, for example a storage device (particularly a reeling device) for the optical fibre.
In greater detail, the tensioning device according to the present invention comprises a fixed pulley carried by a supporting structure and a movable pulley carried by a slide which is slidable along a rectilinear guide carried by the supporting structure.
The optical fibre entering the tensioning device is wound around the fixed pulley for a certain length, and passes from here to the movable pulley around which it is wound for a further length. The distance between the axes of the fixed pulley and the movable pulley is variable to modify the length of the portion of fibre which extends between the two pulleys and consequently the tension applied to the fibre. To vary the distance between the axes of the fixed pulley and the movable pulley there are used, as mentioned above, a fibre tension measuring device, for example a load cell associated with the fixed pulley, and a device for moving the slide which can control the distance between the axes of the pulleys in accordance with the signal supplied by the tension measuring device, to automatically keep the tension in the fibre practically constant. The movement device can comprise an electric motor and a system for transmitting the motion from the motor to the slide, for example a screw and nut coupling in which the screw extends parallel to the rectilinear guide and is rotationally movable by the force of the motor and in which the nut is coupled to the slide, or a belt transmission, for example a toothed belt transmission.
Since it automatically controls the tension of the fibre without using the force of gravity or other predetermined forces, the aforesaid optical fibre tensioning device can be used to modulate in the desired way the tension applied to the optical fibre.
In this way, the movement of the movable pulley is made entirely independent of the force of gravity or predetermined elastic forces, and is achieved by the application of a controllable force.
Thus a tensioning device which automatically adapts itself to any variation of the operating conditions is provided.
In a first aspect, the present invention relates to an optical fibre tensioning device, for use in an optical fibre advance path, comprising at least one movable pulley around which the optical fibre is wound for a certain length at least; the position of the said movable pulley being variable to modify the tension applied to the optical fibre,
and additionally comprising a tension measuring device capable of measuring the said applied tension and of generating a signal indicating the tension, and a motorized device for moving the said movable pulley, capable of receiving the said signal indicating the tension and of moving the movable pulley in accordance with the said signal indicating the tension, to automatically keep the tension applied to the optical fibre essentially constant.
Preferably, the tensioning device also comprises a fixed pulley around which the optical fibre is wound for a certain length at least, the distance between the axes of the said fixed pulley and the said movable pulley being variable to modify the tension applied to the optical fibre, and the said motorized movement device being capable of controlling the said distance to automatically keep the tension applied to the optical fibre essentially constant.
The said motorized movement device may comprise a guide along which a slide supporting the said movable pulley can be moved and an actuator device for moving the said slide along the said guide.
The said actuator device may comprise a screw movable rotationally by the force of an electric motor and a nut connected to the said slide, or an electric motor and a motion transmission system of the belt type (for example, one using a toothed belt) to connect the motor to the slide.
The tensioning device may comprise an electronic control device which receives reference signals and measurement signals at its input, and is capable of generating, in response to the said input signals, operating signals for the closed-loop control of the said motorized movement device.
The said electronic control device is preferably a device for the closed-loop control of the tension applied to the optical fibre, comprising:
a subtractor to which is supplied a tension reference signal, proportional to a target tension of the optical fibre, and a measurement signal, consisting of the said signal indicating the tension of the optical fibre; and
a controller, preferably a proportional-integral derivative controller, receiving at its input a tension error signal supplied from the output of the said subtractor, and generating at its output a drive signal which can be used to operate the said motorized movement device and to control the said distance.
Thus a closed-loop control of the tension present in the fibre is provided, by means of which this tension is constantly monitored, causing an immediate intervention of the movement device whenever the measured tension deviates from the reference tension. The tension in the fibre is thus kept constantly equal to the reference tension, providing a particularly precise control.
Advantageously, the said tension measuring device may comprise a load cell connected to the said fixed pulley and capable of generating at its output the said signal indicating the tension.
Preferably, the said motorized movement device comprises a guide along which a slide supporting the said movable pulley can be moved, and an actuator device for moving the said slide along the said guide; the said electronic control device implementing a system for the closed-loop control of the position of the said slide along the said guide.
Preferably, the tensioning device is connected for operation to a device for storing the optical fibre, capable of receiving the optical fibre from the tensioning device, and the said system for the closed-loop control of the position of the said slide along the said guide comprises:
a further subtractor to which is supplied a position reference signal, correlated with a reference position of the said slide along the said guide, and a measurement signal, proportional to the actual position of the said slide along the said guide; and
a further controller, preferably a proportional-integral derivative controller, receiving at its input a position error signal supplied from the output of the said further subtractor, and generating at its output a further drive signal which can be used to operate the said storage device.
The said storage device preferably comprises at least one motorized reel on which the optical fibre from the said tensioning device is wound, the said motorized reel being rotationally movable about an axis of rotation under the action of an electric motor, and the said further drive signal being usable for the control of the said electric motor.
The said electronic control device preferably comprises an integrator receiving at its input a signal correlated with the speed of movement of the said slide along the said guide and generating at its output the said measurement signal proportional to the actual position of the said slide along the said guide.
In this way, it is ensured that the movable pulley is always kept in the proximity of a reference position from which it is always possible to move the movable pulley in a useful way to compensate for a variation of tension; thus the slide is not permitted to be positioned at a terminal position at which it is no longer possible to move the movable pulley in one of the directions of movement.
In a further aspect, the present invention relates to a method of controlling the tension applied to an optical fibre along a path of advance of an optical fibre, the said method comprising the stage of forming a portion of the said path having a length which can be varied to modify the tension applied to the said fibre, and the stages of measuring the tension applied to the optical fibre and controlling the said length in an automatic way, by means of a motor, in accordance with the said measured tension, to keep the tension of the optical fibre practically constant.
Preferably, the method comprises the stage of closed-loop control of the tension applied to the said optical fibre in accordance with at least one reference signal, proportional to a target tension of the fibre, and a measurement signal which is generated in this stage of measuring the tension and is proportional to the tension to which the optical fibre is actually subjected.
The said stage of closed-loop control of the tension preferably comprises the stages of:
comparing the said tension reference signal with the said measurement signal, generating a tension error signal; and
processing the said error signal, generating at the output a drive signal which can be used to modify the said length by means of a motor and to keep the tension in the fibre essentially constant.
The method preferably comprises the stage of closed-loop control of the position of a movable member in accordance with at least one position reference signal which is proportional to a target position of the said movable member, and a measurement signal which is proportional to the instantaneous position of the said movable member, the variation of the position of the said movable member causing the variation of the said length.
The method preferably also comprises the stage of storing the optical fibre at the end of the said advance path; the said stage of closed-loop control of the position of the said movable member comprises the stages of:
comparing the said position reference signal with the said position measurement signal, generating a position error signal; and
processing the said position error signal, generating at the output a drive signal which can be used to modify the speed at which the optical fibre is stored.
In a further aspect, the present invention relates to an optical fibre processing system, for example a draw tower, an extrusion line or an apparatus for measuring the breaking strength of an optical fibre, in which there is an optical fibre tensioning device capable of receiving at its input an optical fibre from a source of optical fibre, for example an optical fibre traction device, and capable of supplying the optical fibre from its output to a device for storing the optical fibre, for example a motorized reel, the said tensioning device comprising at least one movable pulley around which the said optical fibre is wound for a certain length at least, the position of the said movable pulley being variable to modify the tension applied to the said fibre, the tensioning device additionally comprising a tension measuring device capable of measuring the said applied tension and of generating a signal indicating the tension, and a device for the motorized movement of the said movable pulley which is capable of receiving the said signal indicating the tension and of moving the movable pulley in accordance with the said signal indicating the tension to automatically keep the tension in the fibre practically constant.
Preferably, the system comprises an electronic control device capable of providing a system of closed-loop control of the tension applied to the optical fibre, and comprising:
a subtractor to which is supplied a tension reference signal, proportional to a target tension of the optical fibre, and a measurement signal, consisting of the said signal indicating the tension; and
a controller, preferably a proportional-integral derivative controller, receiving at its input a tension error signal supplied from the output of the said subtractor, and generating at its output a drive signal which can be used to operate the said motorized movement device.
Preferably, the said motorized movement device comprises a guide, along which a slide supporting the said movable pulley can be moved, and an actuator device for moving the said slide along the said guide; the said electronic control device providing a system of closed-loop control of the position of the said slide along the said guide.
Preferably, the tensioning device is connected for operation to the optical fibre storage device, and the said system for the closed-loop control of the position of the said slide along the said guide comprises:
a further subtractor to which is supplied a position reference signal, correlated with a reference position of the said slide along the said guide, and a measurement signal, proportional to the actual position of the said slide along the said guide; and
a further controller, preferably a proportional-integral derivative controller, receiving at its input a position error signal supplied from the output of the said further subtractor, and generating at its output a further drive signal which can be used to operate the said storage device.